Photomultipliers (also called PMT, photomultiplier tube) are electron tubes which amplify weak light signals (down to individual photons) and convert them into electrical signals. Besides individual photomultipliers, arrays of a plurality of photomultipliers can also be used.
Photomultipliers typically have one or a plurality of photocathodes, and also an anode and a plurality of dynodes arranged between the photocathode and the anode. The dynodes and the anode together form a so-called secondary electron multiplier, which is disposed downstream of the photocathode. The photocathode, the dynodes and the anode are usually connected to one another by way of a voltage divider with voltage divider resistors and/or other electronic components such as, for example, transistors or similar stabilizing elements.
Photons impinging on the photocathode have the effect that electrons are emitted from the surface of the cathode (photoemission, photoeffect). These photoelectrons are accelerated in the electric fields of the photomultiplier, and upon impinging on the dynodes generate further electrons until, finally, an electron cascade occurs at the anode. These charges are usually diverted from the anode, for example to ground, wherein this current signal (for example after conversion into a corresponding voltage signal) can be coupled out and utilized as the signal of the photomultiplier.
Typical photomultipliers operate with 10 dynodes. Customary gain factors lie within the range of 105 to 107.
Photomultipliers of this type are used for example as light detectors in modern microscopes such as, for example, optical scanning microscopes. By way of example, these may be fluorescent microscopes, for example microscopes in which a sample is scanned with an excitation beam by use of a scanning device. The sample is thereby excited locally to effect luminescence, wherein the luminescence photons are recorded by the photomultiplier or photomultipliers. As an alternative or in addition, it is also possible for example to detect light beams transmitted by the sample (transmitted-light microscopes). Other types of optical microscopes are also contemplated, however.
Particularly when used in microscopes, but also when used in other types of optical devices, photomultipliers are often faced with the problem of an overload. The overload arises as a result of a predetermined high voltage being applied to the photomultiplier usually by a high-voltage source. The high voltage, and thus the sensitivity of the photomultiplier, are chosen such that under the given light conditions, the anode (wherein a plurality of anodes may also be provided) of the photomultiplier is not overloaded by an excessively high current flow. Thus, a maximum current at which the anode is not yet damaged is usually provided. At currents which exceed the maximum current, damage to the anode can occur, for example as a result of thermal decomposition of the anode material.
Particularly when used in microscopes, however, it often happens that the photomultiplier is exposed to unexpected changes in the light conditions. In particular, these may be changes in the ambient light conditions. By way of example, a photomultiplier of this type can be used at a location in the housing of the microscope at which ambient light can penetrate unexpectedly (for example as a result of the housing being opened), which ambient light would then lead, in the case of the predetermined sensitivity, to an anode current exceeding the maximum current.
One possibility for protecting the photomultiplier would consist in utilizing the photomultiplier signal by way of a corresponding feedback in order to set the high-voltage supply of the photomultiplier to a lower sensitivity. By way of example, the high-voltage supply would be correspondingly reduced in this case. The problem, however, is that controls of this type in many cases have transient recovery times in the region of hundreds of microseconds up to the milliseconds range, which may already suffice to permanently damage the photomultiplier.